on 04-Sep-2020 (Fri)

Bacteria were among the first life forms to appear on Earth

Root nodules are found on the roots of plants, primarily legumes, that form a symbiosis with nitrogen-fixing bacteria.^[1] Under nitrogen-limiting conditions, capable plants form a symbiotic relationship with a host-specific strain of bacteria known as rhizobia

An endosymbiont or endobiont^[1] is any organism that lives within the body or cells of another organism most often, though not always, in a mutualistic relationship. (The term endosymbiosis is from the Greek: ἔνδον endon "within", σύν syn "together" and βίωσις biosis "living".) Examples are nitrogen-fixing bacteria (called rhizobia), which live in the root nodules of legumes; single-cell algae inside reef-building corals, and bacterial endosymbionts that provide essential nutrients to about 10–15% of insects.^[2][3]

We can distinguish therefore in the body of a polyp the column, circular or oval in section, forming the trunk, resting on a base or foot and surmounted by the crown of tentacles, which enclose an area termed the peristome, in the centre of which again is the mouth. As a rule there is no other opening to the body except the mouth, but in some cases excretory pores are known to occur in the foot, and pores may occur at the tips of the tentacles. Thus it is seen that a polyp is an animal of very simple structure,^[1] a living fossil that has not changed significantly for about half a billion years (per generally accepted dating of Cambrian sedimentary rock).

Anatomy of a coral polyp

Saccharomyces cerevisiae reproducing by budding

Budding is a type of asexual reproduction in which a new organism develops from an outgrowth or bud due to cell division at one particular site. The small bulb-like projection coming out from the yeast cell is called a bud. Since the reproduction is asexual, the newly created organism is a clone and excepting mutations is genetically identical to the parent organism.

It is an almost universal attribute of polyps to reproduce asexually by the method of budding. This mode of reproduction may be combined with sexual reproduction, or may be the sole method by which the polyp produces offspring, in which case the polyp is entirely without sexual organs.^[1]

A coral "group" is a colony of myriad genetically identical polyps. Each polyp is a sac-like animal typically only a few millimeters in diameter and a few centimeters in height. A set of tentacles surround a central mouth opening. Each polyp excretes an exoskeleton near the base. Over many generations, the colony thus creates a skeleton characteristic of the species which can measure up to several meters in size. Individual colonies grow by asexual reproduction of polyps. Corals also breed sexually by spawning: polyps of the same species release gametes simultaneously overnight, often around a full moon. Fertilized eggs form planulae, a mobile early form of the coral polyp which when mature settles to form a new colony.

All prokaryotes are unicellular and are classified into bacteria and archaea. Many eukaryotes are multicellular, but many are unicellular such as protozoa, unicellular algae, and unicellular fungi

Unicellular organisms are thought to be the oldest form of life, with early protocells possibly emerging 3.8–4 billion years ago.

Although some corals are able to catch plankton and small fish using stinging cells on their tentacles, most corals obtain the majority of their energy and nutrients from photosynthetic unicellular dinoflagellates of the genus Symbiodinium that live within their tissues. These are commonly known as zooxanthellae and gives the coral color. Such corals require sunlight and grow in clear, shallow water, typically at depths less than 60 metres

corals are increasingly at risk of bleaching events where polyps expel the zooxanthellae in response to stress such as high water temperature or toxins.

Coral bleaching occurs when coral polyps expel algae that live inside their tissues. Normally, coral polyps live in an endosymbiotic relationship with these algae, which are crucial for the health of the coral and the reef.^[1] The algae provides up to 90 percent of the coral's energy. Bleached corals continue to live but begin to starve after bleaching.^[2] Some corals recover.

The leading cause of coral bleaching is rising water temperatures.^[3] A temperature about 1 °C (or 2 °F) above average can cause bleaching.^[3] According to the United Nations Environment Programme, between 2014 and 2016 the longest recorded global bleaching events killed coral on an unprecedented scale. In 2016, bleaching of coral on the Great Barrier Reef killed between 29 and 50 percent of the reef's coral

Coral belongs to the class Anthozoa in the animal phylum Cnidaria, which includes sea anemones and jellyfish. Unlike sea anemones, corals secrete hard carbonate exoskeletons that support and protect the coral. Most reefs grow best in warm, shallow, clear, sunny and agitated water. Coral reefs first appeared 485 million years ago, at the dawn of the Early Ordovician, displacing the microbial and sponge reefs of the Cambrian.^[1]

The Cambrian explosion or Cambrian radiation^[1] was an event approximately 541 million years ago in the Cambrian period when practically all major animal phyla started appearing in the fossil record.^[2][3] It lasted for about 13^[4][5][6] – 25^[7][8] million years and resulted in the divergence of most modern metazoan phyla.^[9] The event was accompanied by major diversification of other organisms.^[a]

Before the Cambrian explosion,^[b] most organisms were simple, composed of individual cells occasionally organized into colonies. As the rate of diversification subsequently accelerated, the variety of life began to resemble that of today.^[11] Almost all present animal phyla appeared during this period.^[12][13]

Sometimes called rainforests of the sea,^[2] shallow coral reefs form some of Earth's most diverse ecosystems. They occupy less than 0.1% of the world's ocean area, about half the area of France, yet they provide a home for at least 25% of all marine species

Cyanobacteria / s aɪ ˌ æ n oʊ b æ k ˈ t ɪər i ə / , also known as Cyanophyta, are a phylum consisting of both free-living photosynthetic bacteria and the endosymbiotic plastids that are present in the Archaeplastida autotrophic eukaryotes.

Cyanobacteria commonly obtain their energy through oxygenic photosynthesis.^[4] The oxygen gas in the atmosphere of earth is produced by cyanobacteria of this phylum, either as free-living bacteria or as the endosymbiotic plastids

Phototrophic eukaryotes such as green plants perform photosynthesis in plastids that are thought to have their ancestry in cyanobacteria, acquired long ago via a process called endosymbiosis. These endosymbiotic cyanobacteria in eukaryotes then evolved and differentiated into specialized organelles such as chloroplasts, etioplasts and leucoplasts.

By producing and releasing oxygen (as a byproduct of photosynthesis), cyanobacteria are thought to have converted the early oxygen-poor, reducing atmosphere into an oxidizing one, causing the Great Oxygenation Event and the "rusting of the Earth",^[13] which dramatically changed the composition of the Earth's life forms and led to the near-extinction of anaerobic organisms.^[14]

Cyanobacteria produce a range of toxins known as cyanotoxins that can pose a danger to humans and animals.

Oxidation is the loss of electrons or an increase in the oxidation state of an atom, an ion, or of certain atoms in a molecule.

• Reduction is the gain of electrons or a decrease in the oxidation state of an atom, an ion, or of certain atoms in a molecule.

The Great Oxidation Event (GOE), sometimes also called the Great Oxygenation Event, Oxygen Catastrophe, Oxygen Crisis, Oxygen Holocaust,^[2] or Oxygen Revolution, was a time period when the Earth's atmosphere and the shallow ocean experienced a rise in oxygen, approximately 2.4 billion years ago

Nitrogen fixation is a process by which molecular nitrogen in the air is converted into ammonia ( NH
₃ ) or related nitrogenous compounds in soil.^[1] Atmospheric nitrogen is molecular dinitrogen, a relatively nonreactive molecule that is metabolically useless to all but a few microorganisms. Biological nitrogen fixation converts N
₂ into ammonia, which is metabolized by most organisms

Nitrogen fixation is essential to life because fixed inorganic nitrogen compounds are required for the biosynthesis of all nitrogen-containing organic compounds, such as amino acids and proteins, nucleoside triphosphates and nucleic acids. As part of the nitrogen cycle, it is essential for agriculture and the manufacture of fertilizer. It is also, indirectly, relevant to the manufacture of all nitrogen chemical compounds, which includes some explosives, pharmaceuticals, and dyes.

Nitrogen fixation is carried out naturally in soil by microorganisms termed diazotrophs that include bacteria such as Azotobacter and archaea. Some nitrogen-fixing bacteria have symbiotic relationships with plant groups, especially legumes

Nitrogen can be fixed by lightning that converts nitrogen and oxygen into NOx (nitrogen oxides). NOx may react with water to make nitrous acid or nitric acid, which seeps into the soil, where it makes nitrate, which is of use to plants. Nitrogen in the atmosphere is highly stable and nonreactive due to the triple bond between atoms in the N₂ molecule.^[7] Lightning produces enough energy and heat to break this bond^[7] allowing nitrogen atoms to react with oxygen, forming NO_x . These compounds cannot be used by plants, but as this molecule cools, it reacts with oxygen to form NO₂ .^[8] This molecule in turn reacts with water to produce HNO₃ (nitric acid), or its ion NO ⁻₃ (nitrate), which is usable by plants

Plants that contribute to nitrogen fixation include those of the legume family

Cyanobacteria are a group of photosynthetic bacteria, some of which are nitrogen-fixing, that live in a wide variety of moist soils and water either freely or in a symbiotic relationship with plants or lichen-forming fungi

Bacteria ( / b æ k ˈ t ɪər i ə / ( listen ) ; common noun bacteria, singular bacterium) are a type of biological cell. They constitute a large domain of prokaryotic microorganisms

Phytoplankton obtain energy through the process of photosynthesis and must therefore live in the well-lit surface layer (termed the euphotic zone) of an ocean, sea, lake, or other body of water. Phytoplankton account for about half of all photosynthetic activity on Earth

Plankton are the diverse collection of organisms that live in large bodies of water and are unable to swim against a current

Essentially, plankton are defined by their ecological niche rather than any phylogenetic or taxonomic classification.

Nekton or necton, from the Greek nekton meaning "to swim", refers to the actively swimming aquatic organisms in a body of water. The term was proposed by German biologist Ernst Haeckel to differentiate between the active swimmers in a body of water, and the passive organisms that were carried along by the current, the plankton